But Colombia_s second city has lately become a medical and business center with a population of 3.5 million and a buddin... But Colombia_s second city has lately become a medical and business center with a population of 3.5 million and a budding tourist industry, its civic pride buoyed by the new public buildings and squares, and exemplified by an efficient and improbably immaculate metro and cable car system. Linking rich with poor neighborhoods, spurring private development, the metro, notwithstanding shrieks elsewhere in Colombia over its questionable construction cost, is for residents of Medell__n a shared symbol of democratic renewal. Even on the rush-hour train I took the other morning, crowds stepped aside to let a cleaning woman with a mop and bucket scrub the floor.

That evening I headed high up into a steep hillside slum where rival gangs still shoot unsuspecting trespassers who cross invisible borders. The city has recently installed an escalator ascending 1,300 feet, much debated at $7 million and disconnected from the rest of the city_s transit network but shortening to a five-minute ride what had been a brutal 30-story climb for some 12,000 residents. I trudged by foot, past armed soldiers, past mothers taking breathers on the decrepit steps that meandered up the mountain, past toddlers on plastic tricycles plunging down vertical streets, to a brightly painted cinderblock hut, a ramshackle aerie overlooking a sprawl of tin houses and open sewers.

The shack is home to Son Bat__, a cultural initiative founded by young black migrants from the Choc__ region of Colombia. Son Bat__ promotes Chocano music and dance, and it benefits from yet another Medell__n initiative: participatory budgeting. Residents here have voted to direct a share of government financing to new schools, clinics and college scholarships. Son Bat__ got to hire music teachers and bought instruments and is adding a new recording studio to its headquarters. A group of players showed me the studio under construction. From another room, music drifted over the barrio and into the warm night air.

I arrived in Medell__n to see the ambitious and photogenic buildings that have gone up, but also to find what remains undone. The murder rate, while hardly low, is now under 60 per 100,000. Architecture alone obviously doesn_t account for the drop in homicides, but the two aren_t unrelated, either. Around the world, followers of architecture with a capital A have focused so much of their attention on formal experiments, as if aesthetics and social activism, twin Modernist concerns, were mutually exclusive. But Medell__n is proof that they_re not, and shouldn_t be. Architecture, here and elsewhere, acts as part of a larger social and economic ecology, or else it elects to be a luxury, meaningless except to itself.

The story of Medell__n_s evolution turns out to be neither as rosy nor as straightforward as fans of new architecture have tended to portray it. It_s generally told as a triumph for Sergio Fajardo, the son of an architect who is the governor of the region and who was the city_s visionary mayor from 2004 to 2007. He pushed an agenda that linked education and community development with infrastructure and glamorous architecture.

But the city_s transformation established roots before Mr. Fajardo took office, in thoughtful planning guidelines, amnesties and antiterrorism programs, community-based initiatives by Germany and the United Nations and a Colombian national policy mandating architectural interventions as a means to attack poverty and crime.